International regulations govern the placement and station keeping for geosynchronous satellites. These regulations require the ground path of a geosynchronous satellite to intersect the equator only within a tolerance window, or "orbital slot", which is allocated to the satellite. Typically, each orbital slot is centered over a specified longitude and is defined about the central position by .+-.0.05 degrees to .+-.0.1 degrees of longitude. Orbital slots currently are separated by minimums specified by their operations frequency band. For example, in the Ka band, orbital slots currently are centered at every two degrees of longitude (i.e., 180 slots exist around the earth). This separation helps to ensure that signals emitted from satellites located in adjacent orbital slots will not significantly interfere with each other.
The finite availability of orbital slots encourages satellite designers to design geosynchronous satellites having the largest possible data-carrying capacity. The capacity of a geosynchronous satellite is typically proportional to the size of the satellite and is limited by the state of current technology. Large, prior art geosyncnronous satellites are expensive to build and place in orbit. Because of the expense, it is not typically feasible to frequently replace geosynchronous satellites which have too little traffic-carrying capacity due to inadequate size and/or outdated technology.
In some prior art systems, multiple geostationary satellites are placed within a single orbital slot in order to increase the traffic carrying capacity of the system within that slot. This is referred to as co-positioning or co-location. For example, multiple geostationary Astra satellites are operated within an orbital slot centered at 19.2 degrees east. U.S. Pat. No. 5,506,780 (Montenbruck, et al.) also discloses a geostationary satellite system which includes multiple, co-located satellites. As used herein, two co-positioned satellites are considered "adjacent" to each other if no other satellites are located between the two satellites. Thus, two satellites are "non-adjacent" when another satellite is located between the two satellites. A satellite located between two co-positioned satellites is referred to herein as an "intervening" satellite.
In order for communication to occur between two ground communication devices which are located within non-overlapping coverage areas of different geostationary satellites, signals must be sent up to the first satellite, then down to a ground station in view of both satellites, then up to the second satellite, and finally down to the other device. The up-down path from a single satellite is called a "bent pipe".
The use of bent-pipe links lead to large signal delays due to the distance between the satellites and ground communication equipment. Because of the large distance between a geostationary satellite and the surface of the earth, a radio signal emitted from a near-equatorial point on the earth incurs a time delay of approximately 120 milliseconds (ms). In prior art systems, the radio signal is translated to another portion of the spectrum (i.e., to avoid interference) and transmitted to the signal's destination. Thus, the signal incurs a total delay of at least 240 ms. When a signal has to be transmitted through multiple bent-pipe geostationary links, the delay increases geometrically.
Signals emitted from ground equipment located at higher latitudes incur even longer time delays because the distance between a non-equatorial point and a geostationary satellite is greater than the distance between a near-equatorial point and the equator. Delays inherent in bent-pipe links make direct communication lis between geostationary satellites desirable.
However, narrow-beam, direct links between non-adjacent, co-positioned geostationary satellites are not possible in prior-art systems because intervening satellites would interrupt the line-of-sight between the non-adjacent satellites, thus disrupting the link.
Besides increased time delays, non-equatorial signals also suffer decreased signal quality due to the curvature of the earth, increased atmospheric considerations, and ground obstacles located along the line-of-sight between the equipment and the geostationary satellite. In fact, above a certain latitude, ground equipment is incapable of communicating with prior art geostationary satellites. Basically, the coverage area of a geostationary satellite is relatively fixed to an area around the assigned longitude. This limitation is unfortunate because areas of the highest traffic demand are not coincident with the equator.
What is needed is a method and apparatus which enables the data carrying capacity within a particular geosynchronous orbital slot to be increased relative to demand and in conjunction with state-of-the-art technology. Further needed is a method and apparatus to decrease signal delay for co-positioned geostationary satellites by enabling non-adjacent, co-positioned geostationary satellites to communicate directly with each other without interruption from intervening satellites. Also needed is a method and apparatus for decreasing signal delay while increasing signal quality for ground equipment which is located at high latitudes and which communicates using geosynchronous satellites. Additionally needed is a method and apparatus to increase the coverage area of geosynchronous satellite systems to include ground equipment at high latitudes which are not currently capable of receiving geosyncnronous satellite service. Also needed is a method and apparatus to selectively provide geosynchronous satellite capacity to non-equatorial geographical areas. Further needed is a method and apparatus for providing global communication coverage using geosynchronous satellites.